1. Field of the Invention
The present invention relates to a semiconductor laser device suitable for use as a light source which is used for long distance, large optical communication. The present invention further relates to a method for fabricating such a semiconductor laser device.
2. Description of the Related Art
In order to realize a semiconductor laser having an improved rapid response property, which is used for long distance, large data transmission, it has been attempted to apply a quantum wire structure to an active layer of a semiconductor laser. The quantum wire structure has a high gain property relative to a quantum thin film structure. In addition, it is theoretically presumed that the semiconductor laser having the quantum wire structure operates with a small current, with high efficiency, and with narrow spectral linewidth (M. Asada et al., IEEE JQE, vol. QE-22, No.9, pp.1915-1921, 1986).
FIG. 6 shows a conventional semiconductor laser device 600 having a quantum wire structure (Arai et al., Proceedings of Electronics Society Conference of Institute of Electronics, Information and Communication Engineers, 1997, pp. 266-267). As shown, double-quantum well active regions 602 each having a trapezoid shape are formed above an InP substrate 601, and the width of the trapezoid is about 35 nm in the center portion thereof. The well regions 602 each have a thickness of 10 nm, and form a quantum wire structure.
A method for fabricating the conventional semiconductor laser device 600 having a quantum wire structure is described with reference to parts (a) to (c) of FIG. 7. As shown in part (a) of FIG. 7, in the first crystal growth process, an InGaAsP light confinement layer 603, a quantum well active layer 604 having two wells, and an InGaAsP protective layer 605 are serially formed on a p-type InP substrate 601. Thereafter, as shown in part (b) of FIG. 7, predetermined portions of the quantum well active layer 604 are selectively etched, thereby forming a plurality of double-quantum well active regions 602 each having a trapezoid shape in a periodic pattern. Thereafter, as shown in part (c) of FIG. 7, an undoped InP layer 606, InGaAsP light confinement layer 607, and an n-type InP cladding layer 608 are grown through a crystal growth process, thereby confining the double-quantum well active regions 602 each having a trapezoid shape.
However, in this fabrication method, it is necessary to once etch the quantum well active layer 604 so that the quantum well active layer 604 has a quantum wire structure, and faces exposed by etching are exposed to a thermal treatment during a subsequent regrowth process. Such a thermal treatment may introduce a defect by etching a portion of the quantum well active regions 602, and the defect may deteriorate an optical property of the quantum well active regions 602. Furthermore, the long-term reliability of the semiconductor laser device may decrease.
Furthermore, the size of the quantum wire structure having a trapezoid shape, which is formed by etching, varies according to even a small variation of the concentration of an etching solution. Thus, it is difficult to fabricate a quantum wire structure having a uniform size.
According to one aspect of the present invention, a semiconductor laser device includes an InP substrate and a multi-layered structure formed on the InP substrate, wherein the multi-layered structure includes at least a plurality of active regions for outputting a laser beam, and the plurality of active regions each are provided in each of a plurality of grooves dented toward the InP substrate.
According to one embodiment of the present invention, the cross-sectional shape of each of the plurality of grooves is a triangle dented toward the InP substrate.
According to one embodiment of the present invention, the plurality of active regions is made of InAsP.
According to another embodiment of the present invention, the active regions are formed in a periodic pattern in a direction parallel to the resonator length direction.
According to still another embodiment of the present invention, the semiconductor laser device further includes: an InP layer; and a light confinement layer made of a semiconductor having a bandgap energy between the bandgap energy of a semiconductor constituting the active regions and the bandgap energy of InP, wherein the active regions are provided between the light confinement layer and the InP layer.
According to still another embodiment of the present invention, the semiconductor laser device further includes a light confinement layer made of a semiconductor having a bandgap energy between the bandgap energy of a semiconductor constituting the active regions and the bandgap energy of InP, wherein the active regions are surrounded by the light confinement layer.
According to still another embodiment of the present invention, the light confinement layer is made of InGaAsP.
According to still another embodiment of the present invention, the active regions each has a multiquantum well structure.
According to still another embodiment of the present invention, a well layer of the multiquantum well structure is made of InAsP.
According to still another embodiment of the present invention, a barrier layer of the multiquantum well structure is made of InP.
According to still another embodiment of the present invention, the active regions each has a size such that a quantum size effect is obtained.
According to still another embodiment of the present invention, the pitch of the active regions is a multiple of (1/(2xc3x97neff)) by a factor of any integer, where neff is the effective refractive index of the multi-layered structure with respect to an oscillation wavelength.
According to another aspect of the present invention, a method for fabricating a semiconductor laser device includes steps of: forming a plurality of grooves in a surface of an InP layer; and thermally treating the InP layer in an atmosphere including at least a gas containing phosphorus and a gas containing arsenic in a mixed state, thereby forming a plurality of active regions made of InAsP in the plurality of grooves.
According to one embodiment of the present invention, each of the grooves is formed in a triangle shape such that a bottom of the triangle is on a surface of the InP layer.
According to another embodiment of the present invention, wherein the InP layer is an uppermost layer of a multi-layered structure.
According to still another embodiment of the present invention, the method for fabricating a semiconductor laser device further includes a step of forming a light confinement layer adjacent to the InP layer, wherein the light confinement layer made of a semiconductor having a bandgap energy between the bandgap energy of a semiconductor constituting the active regions between the InP substrate and the InP layer and the bandgap energy of InP.
According to still another embodiment of the present invention, the step of thermally treating the InP layer includes a step of intermittently providing a gas containing arsenic, thereby forming a plurality of active regions each having a well layer made of InAsP in the plurality of grooves.
According to still another embodiment of the present invention, the semiconductor laser device includes a step of forming a light confinement layer adjacent to the InP layer, wherein the light confinement layer made of a semiconductor having a bandgap energy between the bandgap energy of a semiconductor constituting the active regions between the InP substrate and the InP layer and the bandgap energy of InP.
According to still another embodiment of the present invention, the light confinement layer is made of InGaAsP.
According to still another embodiment of the present invention, the plurality of grooves have a periodic pattern in a direction parallel to a resonator length direction.
Thus, the invention described herein makes possible the advantages of (1) providing a semiconductor laser device having a quick response property and a long-term reliability, and a fabrication method of such a semiconductor laser device; (2) providing a semiconductor laser device having active regions as small as several tens of nanometers which are arranged at a high density, and thus functioning as a quantum wire laser; and (3) providing a distributed feedback type semiconductor laser including periodically formed active regions that function as a diffraction grating, and thus being capable of oscillating with a single wavelength, providing a low noise property, enabling a long distance optical transmission, etc.